The L3 loop is an important feature of the OmpF porin structure, contributing to both channel size and electrostatic properties. Colicins A and N, spermine, and antibiotics that use OmpF to penetrate the cell, were used to investigate the structure-function relationships of L3. Spermine was found to protect efficiently cells expressing wild-type OmpF from colicin action. Among other solutes, sugars had minor effects on colicin A activity, whereas competitions between colicin A and antibiotic fluxes were observed. Among the antibiotics tested, cefepime appeared the most efficient. Escherichia coli cells expressing various OmpF proteins mutated in the eyelet were tested for their susceptibility to colicin A, and resistant strains were found only among L3 mutants. Mutations at residues 119 and 120 were the most effective at conferring resistance to colicin A, probably due to epitope structure alteration, as revealed by a specific antipeptide. More detailed information was obtained on mutants D113A and D121A, by focusing on the kinetics of colicin A and colicin N activities through measurements of potassium efflux. D113 appeared to play an essential role for colicin A activity, whereas colicin N activity was more dependent on D121 than on D113.
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